Common practice in the prior art, for controlling the output of a D/A converter as a function of an externally provided control signal, is to provide a separate attenuator constructed from discrete components or a separate integrated circuit chip. A typical prior art implementation is illustrated in FIG. 1 in which the output (Q/QN) of the D/A converter 10 is connected to a load circuit 11 by a separate attenuator circuit 12 and a cable 14. The load 14 provides a feedback control signal 15 to the attenuator circuit 12 to control the magnitude of the D/A output signal.
The D/A converter 10 of FIG. 1 is shown in greater detail in FIG. 2. It includes a plurality of current sources A1-An. The current sources are similar in construction and are typically weighted by controlling the dimensions of a transistor Q1. Transistors Q2 and Q3 switch the current provided by Q1 into one of two paths under control of the digital input signal. The outputs from the current sources A1-An are combined and provided as Q and QN to the attenuator 12.
Common to all of the current sources is a compensated bias line 20, which supplies a DC bias voltage to the gate ports of the P-channel transistors Q1 of each current source. To keep the desired output load current constant and settable, a closed loop comprised of transistor Q4, voltage VREF, operational amplifier U1 and resistor R1 is employed. Operation is performed by adjusting the current down through the drain of transistor Q4 by the voltage gain of amplifier U1. Amplifier U1 will approximate equal input voltages on its +/- inputs; therefore, the current down through the drain of Q4 is equal to the value of VREF divided by the value of R1.
Because all of the transistors Q1 in the current sources are located on the same silicon as transistor Q4 and (under normal conditions) located in close physical proximity to each other, good matching characteristics can be obtained. Therefore, the physical dimensions of channel width and length will determine the relative current values. The described loop will compensate for temperature and supply voltage changes as well as tolerances added by the manufacturing process.